Here's a look at some of history's worst pandemics that have killed millions

SAN FRANCISCO -- Health officials are warning that the deadly novel coronavirus outbreak, also known as COVID-19, has the potential to become a global pandemic. A pandemic will expose everyone to an infection, and a proportion of the population will fall sick. The word pandemic comes from the Greek word "pandemos," which means everybody. "Pan" meaning everyone, "demos," population.

We broke down the terms you need to know and also took a closer look at the world's worst infections that have killed millions.

What is a pandemic?

According to the World Health Organization, a pandemic is the "worldwide spread" of a new disease. The US National Institutes of Allergy and Infectious Diseases does not use a scientific or definitive definition of what constitutes a pandemic.

What is an outbreak?

An outbreak is the occurrence of a disease in excess of what is normally expected.

How is that different than an epidemic?

An epidemic is more than a normal number of cases of an illness, specific health-related behavior or other health-related events, that is generally confined to a single community or region.

The most severe pandemic in recent history was the 1918 influenza pandemic, often called the "Spanish flu." The pandemic was estimated to have infected about 500 million people, one-third of the world's population at the time, and killed roughly 50 million people worldwide.

Pandemics are part of human history. According to the Centers for Disease Control, there have been at least four pandemics of influenza ("the flu") in the 19th century, and three in the 20th century.

Here's a look at some of history's worst pandemics.

The Spanish Flu

The Spanish flu was an influenza pandemic that spread around the world between 1918 and 1919, according to the CDC. It was caused by an H1N1 virus, with an avian (bird) origin, though it's unclear exactly where the virus originated. The CDC estimates that about 500 million people (or one-third of the world's population) became infected with the virus. It ultimately caused at least 50 million deaths worldwide with about 675,000 deaths happening in the U.S.

The Black Death

The Black Death, was one of the most devastating pandemics in human history, resulting in the deaths of an estimated 75 to 200 million people across Europe and Asia, between 1347 to 1351. The Black Death re-surfaced in London in 1665 for the Great Plague of London, killed 20% of London's population, in just 18-months, an estimated 100,000 people died.

Asian Flu

According to the CDC, the "Asian Flu" began in East Asia in 1957. The influenza virus was an H2N2 strain, first discovered in Singapore. From there, the virus made its way to Hong Kong and to the coastal cities in the United States. Of the 1.1 million people who died of the Asian flu worldwide, 116,000 of them were in the United States.

Hong Kong Flu

The Hong Kong flu pandemic of 1968 originated in China. Caused by an influenza A virus (H3N2), it was the third pandemic flu outbreak to occur in the 20th century, killing one million people worldwide, 100,000 in the United States.

Swine Flu

The "swine flu" occurred in 2009 with a novel influenza virus, H1N1. According to the CDC, the virus was actually first detected in the US, and spread quickly across the US and the world. Between April 12, 2009 and April 10, 2010, there were 60.8 million cases reported, 274,304 hospitalizations, and 12,469 deaths due to the virus. The CDC estimates 575,400 people died worldwide.

HIV/AIDS

Human immunodeficiency virus, HIV, and acquired immunodeficiency syndrome, AIDS, were first discovered in the early 1980s. AIDS was first detected in American gay communities but it's thought to have developed from a chimpanzee virus from Africa in the 1920s. According to the World Health Organization, 75 million people have been infected with the virus since it was discovered, resulting in 32 million deaths worldwide. An estimated 38,000 new HIV infections still happen in the U.S. each year.

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